1. Field of the Invention
The present invention relates to a high temperature reactor, particularly to a device having a means for circulating high temperature gas and working fluid heat-exchanged with the high temperature gas, and further, to steam generators and a circulating system, which are attached to a nuclear reactor.
2. Related Art Statement
As for a pressure vessel used in a nuclear reactor, it is required to use a prestressed concrete reactor vessel (hereinafter referred to as a "PCRV") for a medium-size or a large-size high temperature reactor having an output of 300 MWe or higher due to the size of a reactor core. In the PCRV type gas-cooled high temperature reactor, a device for driving a control rod or the like is provided above the reactor core, and a helium coolant which has come out of the reactor core and has a higher temperature enters from under the tube bundles of steam generators due to the positional relationship thereof.
FIG. 3 shows a conventional high temperature reactor. This reactor consists of a PCRV 33, a reactor core 31, a graphite reflector frame 32, steam generators 34, a heat transfer tube bundle 35, a feed water line 36, a steam line 37, a gas circulator 38, a cross head 39, a top penetration 40, and an auxiliary core-cooling system 41.
This high temperature reactor adopts a so-called downhill boiling system, in which water flowing in from the feed water line 36 to the upper portion of the heat transfer tube bundle 35 at the top of the steam generators 34 is heated into a two phase flow, to form super-heated steam which flows down through the heat transfer tube bundle 35, and is taken out of the bottom portion of the bundle through the steam line 37.
This downhill boiling system is unnatural from the viewpoint of heat flow dynamics, and the stability at the time of a low output is uncertain. Thus it is deemed that countermeasures therefore will be necessary.
FIG. 4 shows a block fuel-type high temperature reactor, in which 51 designates a reactor core, 52 a PCRV, 53 steam generators, 54 a heat transfer tube bundle, 55 and 56 a feed water line and a steam line, respectively, 57 a gas circulator, 58 a top penetration, and 59 an auxiliary core cooling system. In this high temperature reactor, both the feed water line 55 and the steam line 56 are provided at the bottom portion of the steam generators 53, and the downhill boiling occurs at least partially regardless of the flowing direction of the helium coolant in the steam generators 53.
Accordingly, in the conventional high temperature reactor, the working fluid becomes unnatural from the view point of the heat flow dynamics. And, the stability of the steam generators themselves at the time of the low output is uncertain.
When an abnormal state in the conventional high temperature reactor occurs, or the steam generators are operated at a lowered pressure or at a lowered temperature, or operated with the quantity of feed water increased, the fluidal instability takes place, heat transfer performance is decreased, and pressure loss in the tubes is increased. Further, when a feed water pump is stopped and feed water is supplied by a head difference, since the pressure loss in the tubes is increased due to the fluidal instability as described above, then the water pouring capacity is deteriorated.
Furthermore, according to the conventional device described above, the circulation of the gas between the nuclear reactor and the steam generators is conducted by a blower. In order to save the power of the blower, it is desirable to utilize the natural circulating force of the gas. U.S. Pat. No. 4,243,487 is a typical example in which the core cooling method by this natural circulator of the gas is adopted.
This U.S. Pat. No. 4,243,487 discloses a high temperature reactor, in which, in contrast to the downflow-type nuclear reactor, a blower is disposed at the bottom portions of the steam generators and a heat exchange portion for cooling the gas is provided above the level of a reactor core so that when the blower is stopped, the reactor core is cooled by the natural convection or the natural circulation of the gas.
Nevertheless, in this case, since a heat exchange portion for cooling is provided in a hot gas rising tube of the steam generators, the temperature is lowered in this portion and the rising force of the gas is impaired due to the increased gas density and the lowered pressure, whereby the circulation of the gas is limited to require the power of the blower to be large.
U.S. Pat. No. 4,476,089 discloses a nuclear reactor provided with a steam generator, in which a hot gas rising tube is made hollow and a heat exchange portion for cooling is disposed around the tube.
However, in this case too, the top end of the heat exchange portions of the steam generator is lower than a top gas inlet of the nuclear reactor, so it cannot be said that the natural circulating force of the gas is effectively utilized.